1. Field of the Invention
The present invention relates to a technique for creating a color profile (color conversion profile) used for converting a color signal having a certain color reproduction area (gamut) to a color signal for a color-image output device having a more restricted gamut.
2. Description of the Related Art
For example, when image data displayed on an sRGB-compliant color-image display apparatus is output by the color-image output device such as a color printer in which the gamut is more restricted than that of the color-image display apparatus, conversion from an RGB signal (input color signal) to a color signal (output color signal) such as a CMYK signal appropriate for the color-image output device is required.
The color profile used for the conversion is created, in many cases, as a three-dimensional or four-dimensional look-up table in which an sRGB space is divided into cubes of the same type, and an output color signal value is stored at an address of each lattice point (determined by an RGB value).
When such a color profile is to be created, for example, a following procedure is used. That is, after the RGB value at each lattice point in the sRGB space is converted to color data in a perceptual space such as CIELAB or CIECAM02, which is a device-independent color space, and then gamut conversion to the gamut of the color-image output device is performed, to convert the gamut-compressed color data to an output color signal value for the color-image output device, and a look-up table is created in which the output color signal value is stored at an address corresponding to the lattice point.
As well known, sRGB is an international standard of the color space planned by the International Electrotechnical Committee (IEC), the CIELAB is an international standard of a uniform color space determined by the International Commission on Illumination (CIE) and also specified by the Japanese Industrial Standard (JIS), and the CIECAM02 (Color Appearance Model 2002) is a color appearance model issued by the CIE in 2004.
FIGS. 14A and 14B are schematic diagrams for explaining gamut compression in the CIELAB space. In FIG. 14A, the gamut outermost periphery in a certain hue (that is, the outermost periphery of the sRGB space in the hue) of the sRGB-compliant color-image display apparatus is expressed by a solid line, and the gamut outermost periphery in a certain hue of the color image output device is expressed by a broken line. Gamut compression is processing such that it is determined whether an L*a*b* value corresponding to each lattice point in the sRGB space is inside or outside of the gamut of the color image output device, and when the value is outside the gamut, the L*a*b* value is mapped to an L*a*b* value on the gamut outermost periphery (that is, compressed), and when the value is inside the gamut, the L*a*b* value is mapped to the same L*a*b* value (that is, not compressed). Various methods are known, as shown in FIG. 14B, according to how to decide a mapping destination of the L*a*b* value outside the gamut, that is, a method of compressing the L*a*b* value so that a color difference before and after the compression becomes minimum, a method of compressing the L*a*b* value so that luminance before the compression is stored, a method of compressing the L*a*b* value so that chroma before the compression is stored, and a method referred to as perceptual mapping in which the chroma in the gamut of the color-image display apparatus is relatively replaced by the gamut of the color-image output device (for example, see Japanese Patent Application Laid-open No. 2002-252785). FIGS. 14A and 14B depict an example in which a point s on the gamut outermost periphery of the color-image display apparatus is mapped to a point t having the smallest color difference (the smallest distance) on the gamut outermost periphery of the color-image output device.
When the size and the shape of the gamut of the color-image display apparatus and the gamut of the color-image output device are largely different, the degree of freedom in selection of the color on the gamut outermost periphery (for example, the point t) to be associated with the color outside the gamut (for example, the point s) of the color-image output device is large. This means that there is a large difference in correspondence of color data before and after gamut compression depending on a difference of capability of people involved in creation of the color profile, and on differences of the size and the shape of the gamut of the color-image output device. Because suitability of gamut compression directly affects the color reproduction performance of the color profile, it is desired to take measures to restrict such differences.
An effective prior technique to solve the above problem has been disclosed by Japanese Patent Application Laid-open No. 2002-252785, which is a patent application by the present applicant. In the color map creating apparatus shown in Japanese Patent Application Laid-open No. 2002-252785, two-stage gamut compression including an intervening virtual gamut is used. An outline of the two-stage gamut compression including the intervening virtual gamut is explained with reference to FIG. 15.
As shown in FIG. 15, for example, when gamut compression from the gamut of the sRGB-compliant color-image display apparatus (first gamut=sRGB space) to the gamut of the color-image output device (third gamut) is to be performed, a virtual gamut (second gamut) narrower than the first gamut and wider than the third gamut is set. Two-stage gamut compression is then performed such that an L*a*b* value at each lattice point obtained by performing cube division of the sRGB space is prepared as a representative color of the first gamut, these L*a*b* values are gamut compressed to the second gamut, and the L*a*b* values are then gamut compressed to the third gamut. FIG. 15 is an example of mapping of the point s on the first gamut outermost periphery to a point v on the second gamut outermost periphery, and mapping of the point v to the point t on the third gamut outermost periphery. Thus, by including the intervening virtual second gamut, the degree of freedom in conversion from the first gamut to the third gamut (the degree of freedom in correspondence from the point s to the point t) can be restricted.
For example, in the case of the color-image output device using cyan (C) color material, magenta (M) color material, yellow (Y) color material, and black (K) color material, there are demands for reproduction of a specific color, for example, to reproduce primary colors such as cyan and magenta only by one color material, to reproduce secondary colors such as red and blue only by two color materials, and the like.